1. Field of the Invention
The present invention relates generally to technologies for use in designing or locating base stations coupled over a wireless communications link in a cellular mobile system, and more specifically to a method and computer program for effectively suppressing interference at non-targeted base station in an inter-cell wireless communications link.
2. Description of Related Art
When locating base stations in cellular mobile service areas, it is a current practice to previously conduct a computer simulation to determine the number of base stations and the locations thereof considering a variety of factors. These factors includes, for example, transmission power of each base station, the transmission power of mobile terminals located in each prospective service area, a maximum predicted number of calls, cell sizes, radio wave propagation characteristics depending on terrain and man-made conditions, etc.
Such a computer simulation is typically iterated until eliminating dead spots or reducing such spots to an acceptable extent. These simulations may also be carried out when adding a new base station(s) to the existing service areas or when splitting a busy original cell into smaller ones.
In order to attain effective service coverage, it is usual to construct the base stations high on tall buildings or tall poles.
As is known, it is a current practice to directly couple, using copper wires, optical fiber cables, etc., a plurality of base stations to a local control station in a core network. However, the wire or optical fiber connection of each base station to the core network is usually not economical, and sometimes, practically difficult due to severe terrain conditions (for example). One approach to addressing such difficulties is to wirelessly connect a plurality of base stations while one or more base stations thereof are directly connected to the core network via optical cables (for example).
Before turning to the present invention, it is deemed preferable to describe, with reference to FIGS. 1 and 2, a conventional technology relevant to the present invention.
FIG. 1 is a diagram schematically showing one example of a plurality of service areas which are defined by ellipses in this particular case and are respectively covered by core base stations 10(1)-10(3) and wirelessly linked base stations 12(1)-12(14). In the instant disclosure, each of the core base stations such as 10(1)-10(3) implies a base station which is directly coupled to a core network 14 by way of optical fiber cables 16 (for example) and also coupled wirelessly to the adjacent base stations, and each of the wirelessly linked base stations 12(1)-12(14) implies a base station which is wirelessly coupled to an adjacent base station(s). As is known, the core network 14 includes a wire-line network such as a public switched communications network, IP (Internet Protocol) network, the Internet, etc.
In FIG. 1, a small zigzag line 20 schematically denotes communications between a base station and a mobile unit(s) located in the base station service area, while a large zigzag line 22 schematically denotes the inter-cell wireless communications. The radio communications between a base station and the mobile units are irrelevant to the present invention, and thus, the description thereof will be omitted for brevity.
In FIG. 1, there are three inter-cell wireless communication routes.
The base station antenna used for the inter-cell wireless link is a directional antenna such as a parabolic dish antenna, horn antenna, array antenna, etc having directivity in a horizontal direction and also a vertical direction.
However, the conventional cellular mobile system using the inter-cell wireless communications link has sometimes encountered the difficulty that the radio waves directed to an intended base station may undesirably reach a non-targeted (viz., non-intended) base station(s), resulting in inducing interference at the non-targeted base station. The difficulty just mentioned will be described in brief with reference to FIG. 2.
FIG. 2 is an elevation (upper portion of the drawing) and a top plan view (lower portion) of three base stations 20, 22 and 24 which are respectively provided with a pair of directional antennas 20a-20b, 22a-22b and 24a-24b. According to the conventional technique, when designing or locating the base stations, no attention has been paid to the antenna height of a given base station considering those of adjacent base stations.
As shown in FIG. 2, in the case where the heights of the base station antenna 20a-20b, 22a-22b, and 24a-24b are substantially identical, there is the possibility that the main lobe (main beam) 26 radiated from the directional antenna 20b to the antenna 22a is undesirably received at the non-targeted antenna 24a. This is because the main lobe 26 has three-dimensionally beamwidths in a plane normal to the main lobe axis. In other words, the three-dimensional field pattern of the main lobe 26 may take, for example, a shape resembling an airship (dirigible). Accordingly, even if the base station 24 is arranged on the ground in a manner not to be in line, the main lobe 26 may cause undesirably interference at the non-target base station antenna 24a. 